Boosted engines may provide a number of benefits, such as decreased emissions and increased fuel efficiency, when compared to naturally aspirated engines having a similar power output. Furthermore, boosted engines may be lighter than naturally aspirated engines having a similar power output. As a result, vehicle efficiency is increased when engines are boosted by devices, such as turbocharger.
However, turbochargers may experience lag during certain operating conditions, such as tip-in, start-up, etc. The lag decreases the power output of the engine and delays throttle response, impacting vehicle performance and decreasing customer satisfaction. Furthermore, fixed vane turbochargers may only be sized to efficiently operate over limited engine speed ranges, thereby decreasing engine operating efficiency. Specifically, some turbochargers may have a threshold speed below which a compressor provide negligible boost to the engine. Therefore, when the engine is operated below the threshold speed engine performance may suffer.
U.S. Pat. No. 6,418,719 discloses a variable geometry turbocharger (VGT) and control system for adjusting the VGT to generate a desired amount of back-pressure in the engine. Specifically, a difference between a desired and measured exhaust back-pressure is used to determine VGT adjustment in the control system described in U.S. Pat. No. 6,418,719.
However, the inventors have recognized several drawbacks with the VGT system disclosed in U.S. Pat. No. 6,418,719. For instance, the vanes in the VGT may become stuck during certain operating conditions, such as engine braking carried out by the VGT. As a result, the likelihood of engine damage may be increased due to increased pressure at the turbine inlet and increased in-cylinder pressure due to control loss of intake manifold pressure. Furthermore, malfunctioning VGT vanes may also lead to undesirable back-pressure generation. As a result, engine performance may be negatively impacted.
The inventors herein have recognized the above issues and developed a method for operating an engine system. The method includes moving a plurality of vanes in a variable geometry turbocharger toward a closed position in response to tip-out and after moving the plurality of vanes toward the closed position, moving an EGR valve from a fully closed position toward an open position in response to tip-in, the EGR valve in fluidic communication with an exhaust sub-system upstream of the turbine. The method further includes, after moving the EGR valve toward the open position, moving the plurality of vanes based on driver-requested torque, engine temperatures, and/or engine speed. In this way, the EGR may be used to reduce pressure at the turbine inlet. As a result, the likelihood of the turbine vanes becoming unstuck is increased, thereby improving engine operation. Additionally, implementation of engine braking also decreases wear on external braking mechanism in the vehicle, such as friction brakes (i.e., disk brakes), thereby increasing the longevity of the braking system in the vehicle. Furthermore, operating the EGR in this way enables the EGR to assume certain wastegate functionalities, thereby enabling a turbine wastegate to be omitted from the turbocharger system, if desired. As a result, the cost of the turbocharger system may be decreased.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Additionally, the above issues have been recognized by the inventors herein, and are not admitted to be known.